The present invention relates, in general, to lasers and, more particularly, to biasing a semiconductor laser.
Optical communications, Compact Disc (CD) players, and bar code scanners use semiconductor lasers, such as Vertical Cavity Surface Emitting Lasers (VCSELs) and edge emitting lasers. An edge emitting laser typically includes layers of semiconductor material formed on a substrate. More particularly, an edge emitting laser includes a junction formed between two semiconductor materials of opposite conductivity type that are adjacent to each other. The edge emitting laser emits light in a direction parallel to its substrate when electrical current is driven through it. On the other hand, a VCSEL includes an active area disposed between two mirror stacks formed on a substrate. The VCSEL emits light in a direction perpendicular to its substrate when electrical current is driven though it.
In operation, a threshold level of current must be forced through the semiconductor laser for lasing to occur. The threshold level is reached when the stimulated emissions exceed the internal losses and mirror losses. Upon reaching the threshold level, the amount of emitted light rises with the current. In high speed optical transmission systems, it is desirable for the semiconductor laser to have a high operating speed. Since the semiconductor laser needs to reach a threshold level before lasing occurs, a delay is incurred between when the drive signal is applied and when lasing occurs in the semiconductor laser.
Biasing or pre-biasing of the semiconductor laser overcomes the lag time of reaching the threshold level of the semiconductor laser. Determining the extent of the bias is a problem since the threshold level of the semiconductor laser varies substantially as a result of temperature fluctuations and degradation of the semiconductor laser over time. Generally, manufacturers of semiconductor lasers use a back facet photodetector to determine the extent of the bias. This approach is not simple for VCSELs where the back facet is the substrate of the VCSEL, which can absorb the light emitted towards the back facet. Some manufactures form the photodetector underneath the bottom mirror stack of the VCSEL to overcome this problem. However, this requires extra grown layers that increases the manufacturing costs and complexity for manufacturing the devices because of the processing steps needed to contact the photodetector and the bottom mirror stack of the VCSEL.
Accordingly, it would be advantageous to have a method for biasing a semiconductor laser near its threshold level. It would be of further advantage for the method to be compatible with standard semiconductor processes and cost efficient.